The Estimation of Acidity. 259 



Schmidt (22) has discussed the matter of />H in a way which 

 makes the use of this notation as a measure of reaction, more 

 intelligible to the student. He says, " the term />H is given to the 

 exponent of ten taken as a positive number. This is the most 

 rational system, since all values are expressed in the same units. 

 Thus C H = 5.03xl(H can be expressed entirely as a power of 

 ten. 



503=10 a70i ( since log. 10 5 -03 =0-702) 



C H =10 0702 xl0~ 10 



10— 9.298 



Therefore P H = 9 298. 

 .Another example : To find P„ when C H = 0"409 x 10 ~ 7 — 



P H =h)g. 10( — 



PH=log. 10 l-log. 10 CH 



log.^CH^lO-'xlO 1 - 612 



= 8'612 

 P H = 17-388 -10 

 P H =7-388. 



Or, starting from the log. 8-612, one may convert this into 

 -the negative form by subtracting from 0.000 and obtain the 

 required result, thus: — 



0.000 



"8.612 



—7.388 

 Which is the required pH. 



A useful method of calculating the hydrogen concentration 

 decimal points of the pH from right to left as abscissae, while 

 it one draws a diagonal line oh semi-log paper, and plots the first 

 decimal points of the /H from right to left as abscissae, while 

 the hydrogen ion concentrations are plotted, from 0.1 to 1.0 as 

 ordinates. 



If, for example, one wishes to convert />H 6.7 into a corres- 

 ponding hydrogen ion concentration, one finds by examination that 

 6.7 cuts the diagonal line at 0.2, and the concentration is 0.2xl0- 6 . 



By plotting the electrical potentials against the hydrogen ion 

 concentration on semi-log paper one can, if necessary, avoid the 

 ruse of S »rensen's exponent or pH. 



